In display devices such as liquid crystal display (LCD), electrochromic display (ECD), plasma display (PDP), and electroluminescent display (ELD) devices and in metal-insulator-semiconductor (MIS) solar cells, transparent electrodes are generally formed on the surface of a substrate made of glass or the like. A transparent film with low resistance can be obtained using a metal such as silver, but in order to ensure sufficient light transmissivity, it is necessary to form an extremely thin film having a thickness of approximately 10 nm. Difficulties, however, arise in the handling of such a thin film, as such a film can be easily damaged in subsequent steps such as in a step of patterning the electrodes. For these reasons, an oxide conductive material such as zinc oxide, indium oxide, or tin doped indium oxide (ITO), which although has a high resistivity in comparison to metal, has a high hardness and is not particularly susceptible to deterioration in function caused by oxide degradation, is used for the transparent electrodes formed on a substrate. In particular, films made of ITO are widely used because of the low resistivity thereof. In ITO, Sn4+, which substitutes for In3+ in oxide indium In2O3, generates a carrier electron. As is the case with indium oxide, the crystal structure is a cubic bixbyite structure. For the formation of an ITO film, sputtering, which enables the formation of a low resistance film on a substrate with a large area at a relatively low temperature, is generally employed. The characteristics of a film formed by sputtering vary according to the substrate temperature during formation. A film formed at a high temperature of approximately 200° C. or higher is a so-called polycrystalline film made up of an aggregate of microcrystal. In such a film formed at a high temperature, microcrystals in each of certain regions orient in substantially the same direction, thereby forming domains. A temperature of 250° C. has been determined to be the substrate temperature at which a film having both the highest possible transparency and the lowest possible resistance can be obtained. By contrast, a film formed at a low temperature is amorphous or has a structure mainly in an amorphous phase with crystal fine grains being dispersed therein. While an amorphous film has excellent etchability for the processing of electrodes, fundamental characteristics such as conductivity and transparency are inferior to those of a polycrystalline film. Chemical resistance and corrosion resistance are also inferior. In a film that is a mixture of the amorphous phase and the crystal phase, linearity of film patterning is poor because the etching speed for each phase differs greatly. In, for example, cases in which crystal grains are dispersed in an amorphous matrix, it is only the crystal phase that is not etched, remaining as residue and becoming a cause of defects. In addition, because most of the film is the amorphous phase, the film is inferior in terms of a variety of properties such as conductivity, transmissivity, chemical resistance, corrosion resistance, and durability.
For example, in Japanese Unexamined Patent Publication No. 4-48516, it is proposed that, for the transparent electrodes, an ITO film or an indium oxide film, each being oriented in a specified direction and having excellent patternability for etching, be employed. In the same publication, an amorphous film is proposed as an electrode material having excellent patternability for etching. While an amorphous film has excellent etchability, it is inferior in terms of fundamental characteristics such as conductivity and transparency. In Japanese Unexamined Patent Publication No. 8-94230, a randomly-oriented polycrystalline ITO film formed by sputtering at a substrate temperature of 180-350° C. is proposed as a transparent conductive film having excellent etchability.
As in the above examples, polycrystalline films for electrodes have conventionally been formed by sputtering with the substrate temperature fixed at a high temperature of 200° C. or higher. Thus, it was necessary to employ a substrate made of a material capable of withstanding a high temperature of 200° C. or higher during formation of a film, such as a substrate made of glass.
In recent years, many attempts have been made to use a substrate made of a synthetic resin rather than a substrate made of glass, as a synthetic resin substrate has the advantages of being light weight and difficult to break. However, synthetic resin substrates can withstand at most a temperature of 180° C. Therefore, it is not possible to form, on a surface of the substrate, a transparent conductive film made of an oxide such as ITO or zinc oxide at high temperatures of 200° C. or higher. Namely, electrodes formed on a synthetic resin substrate had to be either a film in which the amorphous phase and the crystal phase are mixed or a film in the amorphous phase. In Japanese Unexamined Patent Publication No. 9-61836, a method is proposed wherein, by mixing H2O in a sputtering gas, an ITO thin film having excellent etchability and a low density of crystal grains dispersed in an amorphous matrix is formed on a synthetic resin substrate. This film, however, also is inferior to a polycrystalline film formed at a high temperature in terms of conductivity and transparency.
In Japanese Unexamined Patent Publication No. 5-346575, a method is proposed wherein resistance is improved by using vapor deposition to form an ITO film on a synthetic resin substrate at a temperature below the temperature at which the substrate undergoes a change in shape and calcining the film in air to adjust the degree of oxidization. However, with this method also, fundamental characteristics such as resistivity and transmissivity are insufficient in the resulting film.
The temperature of heating is limited, not only in cases where substrates made of synthetic resin are used, but also in cases where a color filter layer and a light-emitting layer made of an organic material are provided on a same substrate. For example, a transparent conductive film for use as the electrodes of a super twisted nematic (STN) mode color LCD is formed on a color filter made of organic material. The substrate temperature during the forming of this transparent conductive film is limited to approximately 200° C. or less due to the presence of the color filter material.
Thus, there has been a need for a method of forming, at a low temperature, a conductive film that has excellent resistivity, transmissivity, etchability, and the like.
In addition, because of the difference in the thermal expansion coefficient of a substrate and a conductive thin film formed on the surface of the substrate, the substrate is susceptible to bend. When bend arises in a substrate, precision in the processing of the formed conductive thin film into electrodes is reduced. The higher the temperature during formation of the conductive thin film, the larger the bend becomes. In other words, when the film is formed at a low temperature, while bend in the substrate is minimal, a film with sufficient characteristics cannot be obtained. In amorphous silicon (a-Si) solar cells in which a transparent conductive film, an amorphous silicon layer, and an aluminum electrode layer are stacked on a substrate, in organic ELDs in which a pair of electrode layers and an organic light-emitting layer sandwiched therebetween are stacked on a substrate, and the like, precision in the processing of other layers stacked on the conductive thin films suffers. In any case, in order to obtain a high wiring density and a stable internal resistance, it is absolutely necessary that such a bend be reduced. In particular, in a-Si solar cells, which are set up outside, good reliability and durability are required, making bend a serious problem.
Synthetic resin substrates are more susceptible to bend than are glass substrates. For this reason, the formation of conductive thin films on synthetic resin substrates has conventionally been carried out in the relatively low range of from room temperature to 150° C. In order to meet the demand for a reduction in the size, a reduction in the weight, and an improvement in light transmissivity of display panels, it is necessary that the thickness of the substrates be reduced. At the same time, in order to reduce the internal resistance of a circuit, it is necessary that the thickness of conductive films for electrodes and the like be increased. Thus, as reduction in the size and improvement in the performance of devices progress, the more serious a problem bend in substrates and fractures and the like that arise along with bend become.
In Japanese Unexamined Patent Publication No. 2000-222944, a method of forming a low stress and low resistance ITO film is proposed with the aim of preventing fractures in the thin film. In this method, under the condition that the substrate temperature be approximately 200° C., a polycrystalline film made up of an aggregate of polygonal column-shaped crystal grains is formed by ion plating with arc discharge plasma. Although this method is effective for preventing fractures in the thin film, it is not effective for suppressing the bend in a substrate that arises with heating at a high temperature. It is also difficult to use this method for forming a film at a low temperature on a substrate made of synthetic resin. Thus, there has been a need for a method of forming, at a low temperature, a conductive thin film having excellent resistivity, transmissivity, etchability, and the like, that does not also bring about a change in the shape of the substrate.